The present invention relates to microelectronic devices and sensors that have a semiconductor/conducting polymer interface.
The operation of traditional semiconductor devices such as diodes and transistors relies on interfacial properties. Such devices are generally based on interfaces or multilayer structures consisting of conducting (e.g. metals such as Au or Ni), semiconducting (e.g. Si, GaAs) and insulating materials (e.g. SiO2) (S. M. Sze, Physics of Semiconductor Devices, Wiley, New York (1981)). Conducting polymers have been introduced into a number of these devices by simple substitution (see e.g. J. H. Burroughs, C. A. Jones, and R. H. Friend, Nature 335, 137 (1988); J. H. Burrough et al., Nature 347, 539 (1990)). The conducting polymer serves as a replacement for a metal or a semiconductor in traditional device architectures. Although the introduction of a conducting polymer may bring certain advantages in processing and/or chemical diversity, the operational principle of the vast majority of these conducting polymer devices is identical to their more traditional analogues.
There is one notable exception to the simple substitutional approach that has dominated the design of conducting polymer devices. A broad class of devices has been developed whose operation relies on the ability to modulate the conductivity of a conducting polymer through manipulation of its electrochemical potential. This property has served as the basis for electrochemical transistors (see e.g. J. W. Thackeray, H. S. White, M. S. Wrighton J. Phys. Chem. 89, 5133 (1985); E. P. Lofton, J. W. Thackeray, and M. S. Wrighton, J. Phys. Chem. 90, 6080 (1986)) and a wide range of conductometric sensors (see e.g. Pearce et al., Analyst 118, 371-377 (1993), Shurmer et al. Sens. Act. B 4, 29-33 (1991), Y. Miwa et al., Bull. Chem. Soc. Jpn. 67, 2864-6 (1994); A. Talaie, Polymer 38, 1145-1150 (1997); P. N. Bartlett et al. Anal. Chem. 70, 3685-3694 (1998)). Unlike traditional semiconductor devices that rely on the electrical characteristics of interfaces, these devices rely on changes in the bulk electrical characteristics of conjugated polymers. In the case of electrochemical transistors, a xe2x80x9cgatexe2x80x9d potential is used to control the electrochemical potential of a conducting polymer that is one electrode of an electrochemical cell. A minimum of two electrodes, termed source and drain, contacted to the polymer serve to sense the change in conductivity observed in response to the gate potential. Since the gate potential serves to modulate the current flowing (induced by a constant source-drain potential) across the source and drain electrodes, amplification and logic functions become possible. In the case of sensors, the gate is the environment. If an analyte in the gating environment induces a change in the electrochemical potential of the conducting polymer, its presence will be sensed through a change in conductivity.
This disclosure generally concerns itself with hybrid conducting polymer devices that rely on the electrical properties of semiconductor/conducting polymer interfaces and their response to changes in the bulk electrical characteristics of the conducting polymer. The electrical characteristics of the conducting polymer are either actively controlled using external electronics or controlled by analytes in an environment to which it is contacted, either directly or through a mediating layer. In the case of active electrochemical control, a variable barrier or tunable diode results. In the case of control by analytes in an environment, a general electrochemical transducer for sensing applications results. A method of generating semiconductor diodes with specific electrical characteristics is also disclosed.
For the purpose of this disclosure, a semiconductor diode is defined as an interface between a material that conducts electricity and that can support an electric field through the formation of a depletion region (typically but not limited to an inorganic semiconductor such as Si, GaAs or InP) and another electrical conductor (such as but not limited to a metal or conducting polymer). In general, the current-voltage characteristics of such a semiconductor diode are described by the following equation relating current, IPS, to applied potential, VPS:                               I          ps                =                              I            o                    ⁡                      [                          1              -                              exp                ⁡                                  (                                      -                                                                  qV                        ps                                            nkT                                                        )                                                      ]                                              (        1        )            
where I0 is the equilibrium exchange current or reverse saturation current, n is the diode quality factor, k is the Boltzmann constant, q is the elementary charge, and T is the temperature. The ps subscripts indicate reference to a conducting polymer/semiconductor interface. Both Io and n depend on the details of current flow at the interface with the theoretical minimum of n=1 generally considered ideal. Io is given by a number of parameters as described by the following equation, with some loss of generality, for an n-type inorganic semiconductor where majority carrier transfer dominates current flow:                               I          o                =                              aqk            n                    ⁢                      N            C                    ⁢                      exp            ⁡                          [                                                                    -                    q                                    ⁢                                      xe2x80x83                                    ⁢                                      φ                    b                                                  kT                            ]                                                          (        2        )            
where a is the active device area, kn is the surface recombination velocity, and Nc is the effective density of states at the conduction band edge of the n-type inorganic semiconductor, and xcfx86b is the Schottky barrier height. It is noted that the barrier height is at times considered as an effective empirical parameter, but its strict definition relates to the magnitude of the interfacial potential barrier. Herein, we used the barrier height in the empirical sense although at times this is equivalent to the stricter definition.
The Schottky barrier height, xcfx86b, is a central parameter determining the precise electrical characteristics of a diode where majority carrier transfer dominates current flow. Through choice of materials, it is possible to exert control over the barrier height of a diode. However, for many semiconductors, in particular so-called small band gap semiconductors, only a very small level of control is possible (E. H. Rhoderick and R. H. Williams, Metal-Semiconductor Contacts, P. Hammond and G. L. Grimsdale, Eds. (Monographs in Electrical and Electronic Engineering, Oxford Univ. Press, Oxford, ed. 2, (1988), vol. 19). For instance, a series of semiconductor diodes fabricated from clean n-type indium phosphide (n-InP) and the following metalsxe2x80x94Ag, Cr, Cu, Au, Pd, Mn, Sn, Al, and Nxe2x80x94allows the effective barrier height to be controlled over a range of only 0.2 eV (N. Newman et al., Appl. Phys. Lett. 46, 1176 (1985)). The tunable diode disclosed herein allows for extensive control over the effective barrier height of semiconductor interfaces. Furthermore, this control is continuous and, if desired, available in a single tunable device rather than in a series of separate devices. For comparison, an embodiment of the tunable diode based on n-InP allows for the effective barrier to be controlled by more than twice that possible with the series of metals described above and again in a single device if so desired.
Certain disclosed devices can serve as general electrochemical transducers. Such transducers can be interfaced to nearly any sensing scheme that relies on potentiometric detection. Classic potentiometric detection schemes measure the electrochemical potential of a material or the junction potential of an interface by comparing the potential signal at one electrode with a second reference electrode such as a saturated calomel electrode (SCE) (A. J. Bard and L. R. Faulkner, Electrochemical Methods (Wiley, New York, 1980)) Several alternatives to this classic mode of measuring electrochemical potential based on conducting polymers have been developed and have served as the basis for the development of a wide range of sensors for analytes such as, protons, glucose, and organic vapors (see e.g. Pearce et al., Analyst 118, 371-377 (1993), Shurmer et al. Sens. Act. B 4, 29-33 (1991), Y. Miwa et al., Bull. Chem. Soc. Jpn. 67, 2864-6 (1994); A. Talaie, Polymer 38, 1145-1150 (1997); P. N. Bartlett et al. Anal. Chem. 70, 3685-3694 (1998), L. Martin et al., Thin Solid Films 286, 252-255 (1996), J. Cassidy et al., Anal. Chem. Symp. Ser. 25, 309-14 (1986), M. Josowicz and J. Janata, Anal. Chem. 58, 514-517 (1986)). These conducting polymer devices rely on detecting changes in the electrochemical potential of a conducting polymer induced by an analyte either directly or indirectly, for instance through an analyte specific membrane or with the aid of a redox enzyme. In one scheme, electrochemical potential is sensed by measuring the conductivity of the conducting polymer, a bulk measure which is sensitive to electrochemical potential. There is no need for a reference electrode in this potentiometric detection scheme. In a second scheme, the conducting polymer is used as the gate electrode of an inorganic metal oxide semiconductor field effect transistor. The electrochemical potential of the conducting polymer in essence sets the gate potential of the device and thereby controls the current through it. Again, a separate reference electrode is not necessarily required. This latter scheme is a general one that has been used with a wide range of gate materials, not just conducting polymers.
This disclosure describes a scheme for sensing electrochemical potential using a single electrode consisting of a semiconductor/conducting polymer interface. As was the basis for the tunable diode, the current flow across this interface strongly depends on the electrochemical potential of the conjugated polymer. The electrical characteristics of this interface can hence be used as a potentiometric sensor as long as the conducting polymer has a means to equilibrate with the analyte to be sensed. This approach differs substantially from the two sensing schemes described above. Unlike the simple measure of the polymer""s conductivity, the present approach measures the electrical characteristics of a semiconductor interface and has the potential to be much more sensitive. Unlike the conducting polymer modified field effect transistor, the present approach relies on measuring current flow across an interface of which the conducting polymer is a constituent. In the modified field effect transistor, current flow across a wholly inorganic system is measured and influenced by a polymeric or polymer modified gate electrode at a distance.
Semiconductor/conducting polymer interfaces have been reported previously (M. Ozaki et al., Appl. Phys. Lett. 35, 83 (1979); O. Inganas et al., Physica Scripta 25, 863 (1981); O. Inganas et al., J. Appl. Phys. 54, 3636 (1983); Y. Renjuan etal., Synth. Metals 41-43, 731 (1991); Turut and F. Koleli, Physica B. 192, 279 (1993);. M. J. Sailor et al., Nature 346, 155 (1990)). The dependence of the electrical characteristics of semiconductor/conducting polymer interfaces as a function of the electrochemical potential of the conducting polymer has been previously investigated, but for the limited number of interfaces studied thus far, current-voltage properties were found to be largely independent of electrochemical potential of the conducting polymer. Watanabe et al. have measured the electrical properties of press contacted n-Si | poly(pyrrole) interfaces as a function of the electrochemical potential of the poly(pyrrole) (A. Watanabe et al., Macromolecules 22, 4231 (1989)). Here, the only effects observed were due to changes in the bulk resistance of the poly(pyrrole) rather 5 than changes in the interfacial properties of the semiconductor/conducting polymer interface. Frank et al. have measured the electrical characteristics of n-CdS | PMeT (PMeT=poly(3-methyl thiophene)) interfaces while exposing the conjugated polymer to aqueous solutions of redox couples of varying electrochemical potentials (A. J. Frank et al., J. Phys. Chem. 93, 3818 (1989)). Such a procedure essentially results in the electrochemical potential of the poly(3-methylthiophene) equilibrating to that of the redox electrolyte to which it is contacted. Variation of the electrochemical potential of the redox couple and hence of the electrochemical potential of the conducting polymer did not result in any substantial or systematic change in the electrical properties of the buried n-CdS | PMeT interface. It is believed that these studies failed to reveal any dependence on electrochemical potential due to fabrication techniques (in particular the use of press contacts) in the case of Watanabe et al. and due to material choice (the semiconductor/electrolyte combination selected is subject to deleterious photocorrosion reactions) in the case of Frank et al.
In one of its aspects, the present invention concerns microelectronic devices that have a semiconductor/conducting polymer interface where the conducting polymer is electrically contacted in a manner to assure that it remains exposed to the environment. If the environment is an inert electrolyte to which additional electrodes are contacted, the conducting polymer can be electrochemically oxidized or reduced to control its electrochemical potential and a tunable (variable barrier) diode results. This tunable diode is a device that rectifies current like a traditional diode, but unlike traditional diodes, the effective barrier height of the new device can be actively controlled. This control can be an element of an active device or considered a means for fabricating fixed barrier diodes with controlled barrier heights. Alternatively, the environment can contain an analyte to be sensed, either directly or through a mediating layer. The electrical characteristics (e.g. current-voltage or capacitance voltage) of the semiconductor/conducting polymer interface can be used to sense the analyte if it induces a change in the electrochemical potential of the conducting polymer either directly or through a mediating layer. Here, the semiconductor/conducting polymer interface acts as a general electrochemical transducer. The demonstrated dependence of the electrical characteristics of semiconductor/conducting polymer interfaces on the electrochemical potential of the conducting polymer is thus exploited in new devices.